The present invention relates to an improved die and particularly to an improved die to be used on a pellet mill for the pelleting of materials.
Pellet mills to which the present invention relates generally operate by feeding the material to be pelleted to a compression side of the die, over which an extrusion means, usually several extrusion rollers, operate to force the material through die holes and through a discharge side of the die. The continuous stream of material from the die holes is then subdivided to form individual pellets. One type of pellet mill on which such a die is used consists of a vertical revolving die in which the material to be pelleted is fed to the inside of a vertically supported rotating ring die. Fixed rollers are located on the inside of the rotating die which force the material through the die holes in the ring as it rotates. The material is extruded from the outer face of the die and subdivided by stationary knives. A second type of pellet mill is one in which the extrusion rollers are movable and they revolve on the inside of a stationary die which is located in a horizontal plane. They force the material through the die and into contact with cutoff knives that revolve on the outside of the die to subdivide the material.
In either of the above types of pellet mills, or other types in general, it has been recognized that die construction is a critical factor in obtaining desirable pellets as well as high production rates. For example, the thickness of the die, the number of holes in the die, and the surface finish of the die have all been found to affect pellet quality. The effect of some of these factors on pellet quality are generally discussed in the Proceedings of the 1959 Feed Production School on "Pelleting And Related Subjects", published by the Midwest Feed Manufacturers Association, Kansas City, Mo. One factor which is discussed on page 85 of this publication is the need for countersink on the compression side of the die which assists the flow of material into the die hole thereby improving production rate of the die.
The holes on pellet dies of the above types will vary in length depending on the thickness of the die. The die holes will also usually be "straight" or relieved in some fashion. A relieved die hole has been counterbored on the discharge side to provide a slightly larger diameter relief section, thereby reducing the working thickness of the die or the length of the section in which the pellet is formed. The relief section provides flexibility in selecting dies which are thicker, thereby improving die durability, yet enabling one to obtain the desired thickness for the die hole that actually forms the pellet. The difference in distance between the compression and discharge side of the die or overall thickness of the die and the distance of the relief section is referred to as the effective thickness of the die which in turn defines the thickness or length of the working section of the die in which the pellet is formed. The dimensions of the pellet are defined by the length and width of the working section.
There have also been a variety of dies proposed in which the relief section has been modified. For example, a variable relief die or staggered relief die are generally described in U.S. Pat. No. 3,129,458, and a tapered relief die in U.S. Pat. No. 3,391,657. A uniform relief die is often used and with this type of die all of the die holes have the same degree of relief thereby providing uniform relief along the face of the die with a uniform effective thickness for the die holes. While pellet mill die design has been extensively studied, a critical need still exists for a pellet die that is capable of a high production efficiency, including maximum rates of production and energy efficiency yet produces pellets of excellent quality. Usually, one has to make a compromise between rate of production and pellet quality in choosing the die most suited for use with the widest range of materials or formulas which are to be pelleted.